Thermo-Hydraulic Performance Assessment of Al2O3–Mineral Oil Nanofluids for Direct Immersion Cooling of Lithium-Ion Battery Packs Under Fast-Charging

The increasing thermal load associated with fast charging represents a major challenge for lithium-ion battery thermal management, particularly at high C-rates where conventional air cooling becomes insufficient. This study numerically investigates the thermo-hydraulic performance of Al2O3–mineral oil nanofluids for direct immersion cooling of an 18650 lithium-ion battery pack under fast-charging conditions. A three-dimensional transient CFD model is employed to evaluate Al2O3 volume fractions ranging from 0 to 1.0 vol.%, while the effects of charging rate and coolant inlet velocity are systematically examined. Thermal performance is assessed in terms of maximum cell temperature and temperature uniformity, together with pressure drop and pumping power to account for the associated hydraulic penalty. The results demonstrate that increasing the Al2O3 concentration progressively improves battery cooling and temperature uniformity compared with pure mineral oil, although the corresponding increase in flow resistance reveals a clear thermo-hydraulic trade-off. Increasing the charging rate from 2C to 8C substantially intensifies the thermal load, with the maximum cell temperature reaching 340.47 K at 8C, indicating that nanoparticle enhancement alone may be insufficient under ultra-fast charging. Increasing the coolant velocity from 0.05 to 0.20 m/s under 8C operation reduces the maximum cell temperature from 329.61 to 318.37 K, demonstrating the strong contribution of forced convection to thermal regulation. However, the pressure drop simultaneously increases from 23.89 to 114.57 Pa, accompanied by increased pumping-power demand. Among the investigated cases, 1 vol.% Al2O3 at 0.20 m/s provides the strongest thermal performance under 8C fast charging. These findings highlight the importance of considering both nanoparticle concentration and coolant inlet velocity when evaluating the thermal performance and associated hydraulic requirements of dielectric nanofluid immersion cooling systems under high-rate and ultra-fast-charging conditions.